Former spy Sergei Skirpal and his daughter were poisoned by a military-grade nerve agent of a type developed by Russia, Theresa May has told MPs.

REUTERS

The PM said the government concluded it was "highly likely" Russia was responsible for the Salisbury attack.

Russia's ambassador has been asked to explain whether it was "direct action" by the state or due to it "losing control" of its nerve agent stock, according to the BBC.

The Russian Foreign Ministry labeled May's remarks a "fairy tale".

The chemical used in the attack, the PM said, has been identified as being part of a group of nerve agents known as Novichok.

Theresa May said Foreign Secretary Boris Johnson had told the ambassador Moscow must provide "full and complete disclosure" of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons by the end of Tuesday.

"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom."

May said Russia has previously produced the agent and would still be capable of doing so.

The decision to point the finger at Moscow was also based on "Russia's record of conducting state-sponsored assassinations and our assessment that Russia views some defectors as legitimate targets for assassinations", the PM added.

Earlier, asked whether Russia was to blame, President Vladimir Putin told the BBC: "Get to the bottom of things there, then we'll discuss this".